texts: Based on the smallest number of photons that can deliver information about a star, what is the largest size of the universe?

texts:
I was watching a summary video about the Banach-Tarski paradox and it led me to a question.

If I (a) assume the universe is infinite and (b) assume it is possible to receive a single photon of light from any star within it that has a line-of-site to the Earth and (c) assume that the photon has travelled an infinite distance, then such a photon, if enough information could be extracted from it, would prove the universe is infinitely large.

Today we have finite observational and recording capabilities.  I believe (not being a physicist or astronomer myself) that we can't prove the universe is larger than some size, X, because we cannot detect lower than a particular number of photons, Y, to ascertain the distance they travelled.

So, assuming my assumptions aren't assinine (they very well could be), and ignoring our present observational limitations, what is the smallest number of photons from which we could discern information about the distance those photons travelled, and what distance would that represent?


texts:
If Jupiter was orbiting at 1AU, replacing Earth, but everything else in the solar system remained as it is currently, how much would the increased heat from the sun increase Jupiter's radius?

In other words, how much would Jupiter fluff up if it was closer to the sun?

Would that be close enough to classify it as a warm jupiter, or hot jupiter?  or would it still just be a standard jovian planet?


texts: How to prove to my friend that aliens have never visited earth

texts:
I come from a physics background where I don't believe something unless the evidence is shown to me, however I have a friend who believes Aliens have visited earth , which he believes based on conspiracy theories.
My question is: is there any solid scientific reasoning as to why the conspiracy theories aren't true. As for him I need to give him evidence as to why they don't exist , and not the other way around .


texts: Is there a limit to the distance of detectable gravitational wave events?

texts:
This is somewhat a follow up question to this one.

This article indicates that the binaries detected have a distance between 320 and 2840 Mpc.

With the current technology, is there a limit to the distance of coalescing compact binaries that can be detected through gravitational waves? Or could we potentially see these mergers up to the edge of the cosmos (if there were any)?


texts: What is the force that changes a satellite’s inclination when orbiting a planet with an equatorial bulge

texts:
I tried looking this up without success and I imagine there's a formula for it, with some ridiculous numbers like it drops off to the 5th power of the distance or maybe even more.

Imagine a planet like Jupiter, large equatorial bulge and imagine an artificial satellite orbiting in a polar orbit, not an equatorial orbit.    Jupiter's enormous equatorial bulge should disrupt the satellite orbit and in time (perhaps a very long time), turn it into an equatorial orbit.

Is that correct or incorrect thinking on my part and is there a formula that applies?    I imagine that some kind of measure for equatorial bulge would be needed as well and I'm not sure how that would be measured in a gravitational field from orbit.


texts: Why are CMB peak heights sensitive to the physical densities

texts:
A very simple question: why should it be that the CMB power spectrum allows constraints to be placed on the combination of parameters
$$\omega_c = \Omega_c h^2$$
$$\omega_b = \Omega_b h^2$$
as opposed to $\Omega_b$ and $\Omega_c$? I understand roughly where the constraints come from in terms of matter-radiation equality determining the scale at which modes have some extra radiation driving determining the total matter density in the early Universe and the Baryon density determining the ratio in peak heights but my question is why is it the physical densities that are constrained- why should the $h^2$ factor need to be included here?


texts: Are we seeing a conjunction these mornings?

texts:
I have a doubt about planetary conjunction.

These days, we can see Saturn, Mars, Jupiter and the Moon close to each other.

I mean if Saturn, Mars, Jupiter and the Moon were in conjunction with each other, last February, 19th, 2020.

Is this a conjunction or is it an alignment?


texts: How is the cost of JWST distributed on different parts of development and operations?

texts:
The James Webb Space Telescope costs nearly 9 billion dollar. What parts of this project cost how much?

I suppose that almost every component has to be developed and is not available to buy off the shelf. So most of the costs should end up as salaries to engineers. I see that the top 10% US specialist aerospace engineers earn 163,000 dollar. The JWST budget covers 54,000 such yearly salaries. Are really thousands of the best engineers working on this project? Or are maybe some materials, like beryllium, very expensive? Like JWST's average cost of about $1½ million/kg. Launch costs are anyway negligible in this context.


texts: Is the universe much like the earth, in that if I were to travel in one and only one direction, I'd eventually end up where I started from?


texts: If the Sun's trajectory were altered, would the solar system follow?

texts:
I was reading a text state that the sun is not stationary:

"it orbits the Galactic center once approximately every 226 million years as the galaxy spins which is also traveling through space towards an unknown source of gravity"

So does the solar system will also move when the Sun moves? Does it affect the stability of solar system? I am not much known to the cosmology so if my question is wrongly framed, please let me know.


texts: We quote that the Earth is about 4 billion years old. However earth started with billions of cigarette smoke sized particles. What event or size or something do we use to say this is the start of Earth? Can we know how long the formation from dust to current size took?


texts:
My understanding of telescope mirrors is that they are (generally speaking) a slice out of an imaginary sphere. The center of this sphere is the focal point of the telescope, where the detector is placed. The bigger the sphere, the further away the focal point will be.
With this in mind, why are telescope mirrors nearly flat? Would it not make sense to instead use the same diameter mirror, but with a smaller imaginary sphere so the detector doesn't have to be so far away?
I have made a crude image to demonstrate my point. The blue arc and point represent a more traditional "nearly flat" mirror, whereas the red arc and point represent a mirror that has the same diameter but with a smaller sphere and therefore closer detector.


texts:
I have heard that the JWST will be capable of looking at the early stages of the universe. However I have also heard that it won't be capable of detecting the Oort Cloud that supposedly provides comets for the solar system.

How is the JWST able to see millions of light years away but cannot see a giant spherical cloud of rocks around the solar system?


texts: According to the NASA announcement and the Wikipedia article on TRAPPIST-1, the planets around the TRAPPIST-1 star are named TRAPPIST-1 b through h. What happened to "TRAPPIST-1 a"?


texts: Is there a theory / equation showing whether or not two passing bodies will go into orbit around each other?

texts:
I am looking for a theory that shows whether or not two passing celestial bodies will go into orbit.

I assume there would have to be a critical point where the gravitational attraction is stronger than the inertia of the moving bodies.

The bodies would then orbit until they collide, or be affected by a force to fall behind the critical point and escape orbit.

I am looking for a theory that states that objects past this critical point of gravitational attraction must either collide immediately, or go into orbit after falling within a certain distance.


texts: Laser Collimator producing oval not circle on the Primary

texts:
I own an orion laser collimator which I had not used in a very long time. I decided to collimate an old Celestron NextStar 114 and I realized that the laser did not produce a circle but an oval.
Apparently the telescope is roughly collimated using a collimation cap.
There seems to be a corrector less in the focuser which I don't dare to disassemble. Could this oval be produced by this corrector or perhaps the laser collimator needs collimation too or maybe there is a problem in the secondary mirror?


texts: What would be the apparent magnitude of Betelgeuse if it were as close to Earth as Sirius?

texts: How large and bright would Betelgeuse appear if it were as close to Earth as Sirius, before and after it goes supernova?


texts: Why do comets come from our local Oort cloud instead of from interstellar sources?

texts:
Comets form where stars form. And they are then ejected by the orbital changes of giant planets. But why are they ejected to form specifically a local Oort cloud instead of going interstellar and becoming vagabonds? The fact that comets come from all directions doesn't discriminate between them coming from an Oort cloud or from interstellar sources.

The main argument against the idea of one star capturing a comet from another star, seems to be that the relative velocity is too high for that. But consider the huge age of the Sun which is now on its 18th or so orbit through this crowded galactic star field disc. And which by chance or co-formation might have now, or at some earlier time, roughly the same velocity as some other millions of other stars. And what if there are many (many) more comets formed and/or ejected from young stars than currently supposed?

Is it really unlikely that the Oort "cloud of comets" instead largely represents an interstellar source?


texts: Will the conditions that allow life to evolve still be around in a trillion years?

texts:
Some billions of years ago, systems like the Earth couldn't emerge and therefore life couldn't arise, because not enough of the heavy elements had formed yet.

Assuming there'll neither be a big crunch nor a rip, stars will still be forming in a trillion years.

So the sun is one of the earlier stars in the lifetime of the universe.

It seems weird to be living at the very start of the era of the universe in which life can evolve.
If a conscious being was chosen at random, the odds of it being on one of the first planets that allow life to evolve seems very slim!

So I'm curious about whether some evidence indicates that life will be less likely to emerge later in the universe.

Will stars of a very late generation (the sun being a 3rd generation star)) be similar to the sun, keeping the possibility open for earth-like planets to form?


texts: What do 5-sigma, r, and lowercase-delta mean in the physical descriptions of major telescopes?

texts:
The typical 5σ point-source depth in a single visit in r will be ∼24.5 (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg2 with δ<+34.5∘, and will be imaged multiple times in six bands, ugrizy, covering the wavelength range 320--1050 nm. About 90% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg2 region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to r∼27.5.

I believe 'five-sigma' refers to standard deviations in a Gaussian distribution (statistics), but am unclear what it has to do with a 'point-source depth'.
And I am even more confused by 'r' and 'lowercase-delta'....


texts: How does the Gamma Ray Burst that occurred when 2 black holes merged compare to other GRB's?

texts:
A Gamma Ray Burst was detected 0.4 seconds after the gravitational wave event, GW150914, caused by a black hole merger, and it was in the same part of the sky. It is uncertain whether that Gamma Ray Burst was associated with the black hole merger. The odds of a GRB being coincident (or just background noise) is 0.22%. That implies a 99.78% chance the black hole merger was related to the GRB. Later analysis suggests the GRB was just a background event that occurred in the same place in the sky at just 0.4 seconds after the black hole merger, and therefore not related.

While a GRB may have relativistic jets beaming out from it in opposite directions, research "excludes the possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer." I interpret that to mean this GRB did not have relativistic jets, but was omnidirectional. (Not sure how astronomers came to that conclusion.)

Anyhow, whether it was coincident or not, I am asking about how the energy output of that GRB compares to other GRB's. The Wikipedia article in the link says the "energy emission in gamma-rays and hard X-rays from the event was less than one millionth of the energy emitted as gravitational waves."

How much energy does a typical GRB emit? What was the spectrum spread of a typical GRB? Over what amount of time does a typical GRB emit that energy? (How many seconds?)

How do the energy levels, spectrums, and time durations of other GRB's compare to the GRB associated with the black hole merger?

If it is similar to other GRB's, that would support the hypothesis this GRB was just a coincident event at nearly the same time and location in the sky.

If this GRB has different energy emissions and time duration than other GRB's, that would support the hypothesis it is truly associated with the black hole merger.

When you answer, please provide data, citations, or quotes from original research. I am not looking for unsupported speculation, but real analysis supported by real data.


texts:
What are the velocity, mass, and charge distribution of the solar wind.

Near the earth within the magnetosphere in the ecliptic
Near the earth but outside the magnetosphere in the ecliptic
Outside the ecliptic at 1 AU

I would like to understand the energy content of the solar wind, and how it compares with the solar radiation(solar constant is $1360\, Wm^{-2}$)?


texts: what/how do we mean/derive statistical equilibrium equation of certain energy levels?

texts:
While reading the research paper "Excitation of the Fe XIII Spectrum in the Solar Corona" by D.R. Flower and G. Pineau des Forets in the summary I found the concept of statistical equilibrium equation hard to understand.
Please explain me the ways in which I can obtain these equations and what is the physical meaning of these?

  Solution of the Statistical Equilibrium Equation

  In coronal conditions, the full statistical equilibrium equations need be solved for only the first few ($i=1 \text{ to } m$) energy levels. More highly excited states ($k=m+1 \text{ to } n$) contribute only through the indirect process of electron collisional excitation followed by radiative cascade. In the case of Fe XIII, we solve the statistical equilibrium equations for the states of the ground $3s^2 3p^2$ configuration ($m=5$) and include the indirect contribution of the states of the excited $3s 3p^3$ and $3s^2 3p 3d$ configuration ($k=6 \text{ to } 27$). The error committed in neglecting all other bound states of the ion and the continuum is considered separately later in this Section. In this approximation, the statistical equilibrium equations of levels $1 \text{to} m$ may be written simply as

  $$\sum_{j=1}^{m} d_{ij} N_j=0 \tag{6}$$

  with $d_{ji}$ defined by…(several long expressions)

http://articles.adsabs.harvard.edu/full/1973A%26A....24..181F
http://articles.adsabs.harvard.edu/pdf/1973A%26A....24..181F
Bitcode: 1973A&A....24..181F


texts: Hulse-Taylor binary pulsar - what is the rate of mass/energy loss from the source?

texts:
Following on from an earlier question about the very interesting Hulse-Taylor binary pulsar.  The high-frequency (radio) beam from the spinning pulsar sweeps across Earth about 17 times per second.  The total power of the gravitational radiation (waves) emitted by the binary system is calculated from GTR to be $7.35 × 10^{24}$ watts at present (declining as the orbital period and radius diminish).

Weisberg & Taylor, 2004 report that the beam has "a flux density of about 1 mJy at 1400 MHz."  (mJy = milliJansky, thanks Stan Liou). Assuming that this flux is uniform across a conical beam with cross-sectional radius 5 arc degrees  and assuming that the source is 21,000 light years from Earth and has mass = 1.44 Solar Masses:-  How much energy is emitted (per second) from the source in the beam? Also (if it is possible to estimate a reasonable range of values) what might be the rate of steady mass loss from such a source?


texts: How often do comet occultations of stars happen?

texts:
By travelling to specific places, astronomers can see three thousand star comet transition events every year at m15 for the major (50km+) comets and asteroids averaging about 20 seconds, perhaps that's 25k transitions if they listed 2 seconds occultations by the same 50km comets, perhaps that's not an exhaustive survey, it may be 5/10/20k?

Some easy and fun maths:

There are about 500 >50km comets, and 100k >5km which would transition visibly at 40AU's, so that's 600,000 total eclipses of

There are 1000,000 >1km comets that cause eclipses of average m20 stars, and there are 100 times more m20 stars than the m15 ones, that's 60mn star dimming events at m20.

Conversely, We can perhaps divide that number by 100 (difficult figure to know) for a static view, but you would have to run very fast with your telescope to see 60 million star dimming positions every year, Can we keep the figure at 60mn or is it logical to change that to 0.6?

I must have misjudged it by 1-2 orders of magnitude, multiple times less for velocity or something. Can we surmise if it is 60k or 60million at m15 and m20?

If a comet of 11km at 40AU's travels through the disk of the galaxy, it can hide stars for at least 0.5 seconds. If the stars are N times distanced than the comet's diameter from a perspective, the comet would hide two stars every NxN frames, so for N = 5, a different star would dim every 25 frames. What is the real maths to say what the figure N is at the disk of the galaxy for something like Halley's Comet at m15 and m20?

this page sais: http://www.asteroidoccultation.com/asteroid_help.htm#hgm
In reality there are probably hundreds of asteroid occultations every night.  However many cannot be observed for practical reasons (e.g. star too dim, event duration too short, etc.).


texts: Questions about amateur astrophotographer Nik Szymanek's telescope

texts:
The 2011 Sixty Symbols video Spy Satellites (from Deep Sky Videos) shows amateur astrophotographer Nik Szymanek and his telescope.

Question: Can someone identify the model and design of this particular telescope and it's mount, and explain why there are so many baffles inside the tube (isn't this more than typical?) and the structure of the tube? Is it some composite? Why the spiral pattern?

click all images for full size


texts: How can I determine line luminosities from equivalent width measurements?

texts: I have a set of equivalent width measurements for different emission lines. How can I use them in order to determine the corresponding line luminosities?


texts: What is the time/size/rigidity ratio for a space object to become mostly round?

texts:
In the answer here multiple comets are identified that do not form a single round body.

A drop of water will form a round shape in the time it takes to free fall a couple of feet.

All of the moons and planets in our solar system are essentially round.

Based on observable data, I assume that there is a relationship between the size, rigidity and the passage of time; that will result in all objects subject to only their own gravitational influences (Given: no body is ever truly uninfluenced by others) becoming spherically shaped (round).  Any object that is in microgravity and not round, therefore must be either very rigid and/or very newly formed.

What is the ratio or formula for calculating any one factor when the other two are known?

Note There would be other factors as well, speed of rotation, elasticity and energy of areas moving in opposition.  But I think these (and others?) would be minor compared to the other three.


texts: Is light emitted from a heavier body more redshifted than light from a lighter body?

texts: Is light emitted outwards from, say, a region very close to a black hole more shifted than light with the same wavelength that began next to a less massive object?


texts: 3000th Question: What does the future of astronomy look like with the next generation of telescopes?

texts:
What will the benefits and advantages be of the upcoming generation of telescopes such as LSST, E-ELT and JWST? And more broadly where will this leave astronomy in 10-20 years time?

This isn't a direct duplicate of (What are the next planned space telescopes?) because I want to know what the aims and benefits of such telescopes are, and what new ones might have been planned since.


texts: What is the most populated/numerous stellar system in which the orbits of all objects are known?

texts:
I posed a question or two on here a couple months or so ago about the orbital patterns of each of the seven stars in the two septenary star systems Nu Scorpii and AR Cassiopeiae. They remain unanswered so I have to assume the answer is unknown. So, now I want to what the most numerous star system is for which we actually do know the orbital pattern, and obviously I want to know the orbital pattern.

I am NOT talking about a star with multiple planetary or other smaller bodies. I am talking about multiple STAR systems. Do we know the orbit for systems of multiple stars. Again, I'm not talking about every rock larger than a school bus. I mean STARS. Stars that have nuclear fusion occurring.


texts: Is there any official star gazing location list?

texts:
I recall when I was a child (many years ago) my father taking us out to gaze at the starts. I remember being able to see the Milky Way with our naked eyes (North Central New Jersey). Nowadays living down here in Maryland I can count the stars on my hand because of all of the light pollution.

So I ask, is there some sort of listing that shows where the best places are to gaze without a telescope up at the stars?

I'd like to see the Milky Way again and not the white glow of the city lights.


texts: How did the Ancients understand the ecliptic using the Ptolemaic system

texts:
I studied the Farnese Atlas and I wonder how did the Ancients discover the ecliptic, the equator, etc. with the Ptolemaic system namely without knowing that the Earth rotates on itself, especially for the Ecleptic.

I ask the question because if in their model the Earth does not rotate, so how do they explain the two revolutions of the sun around the earth:

The first one being the daily cycle (sunrise and sunset)
The second one being the tropical year

With a model where the Earth does not rotate, I do not understand how they found out that the Sun make a revolution of a one year around the Earth (whence the ecliptic) with a second daily revolution. How could they conjugate these two Sun's cycles?


texts: Can an astronomical telescope view objects on Earth?

texts: I am new to telescopes and I'm planning to buy an astronomical telescope like the "Celestron AstroMaster 114 EQ Reflector". Just curious to know whether it can be used to view objects on Earth (e.g. traffic and people miles away from tall buildings). Telescopes are used to observe stars; they must have higher power than binoculars. Right?


texts: What's the exact story of jupiter and our earth, not becoming a super-earth?

texts:
I've been watching the Cosmos documentaries by Neil DeGrass Tyson and the ones from Brian Cox, and in one episode (I don't remember from which series) there's something about the creation of the rocky planets in our solar system I don't understand. Let me first clarify what I understood from the explanation:
At the beginning of our solar system, there were lots of gasses and dust, all over the solar system. Some of those coalesced into planets, who became bigger and bigger by attracting more gasses and dust. (That was also what I learnt as a child)
However, when looking at exoplanets, there seem to be quite some "super earths": rocky planets, being quite larger than the rocky planets in our solar system. This created the question why the rocky planets in our solar system stopped growing, and then, a theory, involving Jupiter was formed:
Apparently, Jupiter, being a big gass giant, spiraled towards the Sun, and being so heavy, Jupiter worked like a vacuumcleaner, sucking up the gasses and dust from between the rocky planets, and like this stopping the growth of the rocky planets.
The spiraling movement of Jupiter was stopped, thanks to the formation of Saturn: that large planet pulled Jupiter back from further spiraling towards the sun, and like this, the planets in the solar system got, more or less, their current size.
When I first heard this, I thought "Ok, that's an interesting theory", but later, I remembered something from my years at university:
During my studies (I studied mathematics and mathematical physics), we have proven that the gravity, caused by a sphere with a mass, is exactly the same as the gravity, caused by a single point with the same mass.
This gives me the following idea (about the creation of Saturn and its impact on Jupiter): first there was a ring of gass, floating outside the orbit of Jupiter. Then, some of that gass coalesced and formed Saturn. The mass of the ring and of the resulting planet are the same.
So, the gravity, caused by that ring of gass, and the gravity, caused by Saturn as a coalesced planet, should be equal. Hence this could not have had any impact on Jupiter's movement, so the explanation of our rocky planets, not turning into super-earths because of Jupiter, spiraling towards them and being stopped by Saturn's creation, is wrong!
There are three possible solutions to this:

Either the theory is wrong indeed, but I would really be surprised that I, as a simple guy, am able to find an error in an official theory that simply.
Either I missed something in the explanation. In that case, can somebody tell me what the correct explanation is?
Either my reasoning is wrong: indeed a sphere and a point with the same mass have an identical gravitational impact, but my extrapolation towards a ring of gass and a coalesced planet is incorrect. In that case, can anybody point out the flaw in my reasoning?

Thanks a lot for reading and answering this (I realise it's a long one).


texts: Any record of the Earth passing through the tail (not trail) of a comet?

texts:
Discussion below the Space SE question How hard is it to fly through the tail of a comet? Has it been done? has led me to ask if there is any record of the Earth passing through the tail of a comet.
That's tail not trail. The annual meteor showers we talk about, even the unicorns are the Earth passing through the trail of breadcrumbs distributed long ago. This question is about the tail(s) of comets, those things we see in photos and sometimes by eye.


texts:
I need help with an exercise in the book that I can't tackle. I need to calculate how much time we have to intercept and eliminate a NEO. I got the following orbital parameters:

a=-2791.44 km (semi-major axis),
i=22.3 deg /Hyperbolic),
e=2.77
v=247.96 deg,
r= 2,106,101.331 km (radius from Earth's center of mass)

My problem is that I don't know where to begin, but I did an attempt. I assume that the intercept with the target only can happen when the phase angle between Earth and the NEO are the same.
I know that one can calculate the velocity of the S/C with ($m_y$=gravitational param.):
$$v=\sqrt{(2m_y/r)-(m_y/a)}$$
If we calculate the escape velocity of the Earth with $V_e=\sqrt{2GM_e/r}$, we will get the difference in velocity. With this information, we can calculate the time needed.  Anyone that can help me?


texts: What is the duration of the era during which it will be possible to perceive a total solar eclipse?

texts:
Total solar eclipses occur because the Moon and the Sun have the same apparent size in Earth's sky — the Sun is about 400 times wider than the Moon, but the Moon is about 400 times closer.
It's my understanding that the Moon is currently receding from the Earth at about the rate our fingernails grow (about 1.6 inches, or 4 centimeters, per year). Lunar retreat rates in the distant past may have been slower than this.
At some point, the Moon will no longer be able to fully block the Sun. (This question on StackExchange addresses the period from now onwards; one solution offered is that the Earth will have its last total solar eclipse in about 600 million years.)
However, what I'm interested in is determing the total duration of that period when it will ever be possible to perceive the Sun's corona (and other effects such as Baily's beads) from Earth by means of a total solar eclipse. This requires knowing both how long it is until the last eclipse will occur (~600 million years) and the time that has elapsed since the first such event happened. Note that the critical period I'm trying to discover is that window of time during which the apparent sizes of the Sun and the Moon, as seen from Earth, are approximately the same size, as they are now.
(In case anyone's interested, my reason for asking this is to try to determine the probability that we humans should have evolved at that point in time when it's possible to perceive a total solar eclipse, to further a pet theory of mine that suggests that it may not be a coincidence.)


texts:
I would like to use the dataset for this kaggle but the competition was private, is closed and I have been unable to contact anyone directly.
The challenge is machine learning for photo-z estimation from u,g,r,i,z magnitudes. Out of curiosity, I would like to apply an ML technique not apparent in code samples submitted to the competition.
Photo-z estimation is the process or method of estimating the redshift (z) of an object based solely on its brightness, or magnitude, through various colour filters (u, g, r, i, etc.). This is desirable because sky surveys typically collect all such magnitudes for distant* objects automatically; this is quick and efficient, whereas measuring the spectroscopically red-shift requires particular attention, i.e. more effort and more time. Consequently, photo-z estimation can provide more redshift values for a given investment of effort, which is important for generating observational statistics.
(* We are talking about cosmologically distant here, objects nearby, in our local group of galaxies for example, don't have a "cosmological redshift)
Does anyone know whether the data set is available elsewhere?


texts: How to find a Cepheid's pulsation period using its graph?

texts:
Basically, I've been seeing these graphs that say: Cepheid Period-Luminosity Relation and the Period axis is supposed to be "logarithmic";

How does this graph work? Since the luminosity of a star doesn't just increase, as we know, but it increases and decreases periodically, why does the magnitude keep raising?
How do I find the pulsating period of such a star through a graph like this?


texts: Supernova in Hertzsprung–Russell diagram

texts:
If you could put a supernova in the Hertzsprung–Russell diagram. Where would it be? Would it be near the white dwarfs or near the super giants?

I know that a supernova isn't a star but rather the explosion of a star, but if you technically could. Where would it be located?

Thanks!


texts: At what distance does Proxima Centauri become visible to the naked eye?

texts: The red dwarf star Proxima Centauri is the closest-known star to the Sun (being about 4.2 ly or 1.3 pc away), however it's not visible to the unaided eye for it is too small and dim. I'd like to know how close one should get to see Proxima well, like most other stars. In Celestia the star would become visible to me at about 0.75 ly away. Is Celestia right?


texts: Are there any stars that orbit perpendicular to the Milky Way's galactic plane?

texts:
Most stars orbit in the Milky Way's galactic disc. But is it possible for one to orbit perpendicular to it?
Here on Earth since we're inside the galactic plane we can't get a good view of what the Milky Way looks like. But would the whole Milky Way be visible from a planet orbiting such a star?


texts: why was the probability of one in 10 billion per planet considered highly pessimistic by a recent study on aliens?

texts:
A recent article about the existence of extraterrestrial life claimed that "unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first."
It continues to say that "a probability for civilizations to form of one in 10 billion per planet was considered highly pessimistic."
Here's a link to the actual study. The author of the article is a coauthor of the study.
Could someone go through the study and explain the source of the probabilities and their assessments (optimistic vs pessimistic.)


texts:
My question is simple:

  Is the Solar system stable?

You can see this Wikipedia page.
Edit: Sorry, because i think my question is more about mathematics and classical mechanics of planets in billion years scale, than astronomy. But i think this is our fortuity that Earth didn't doomed in past billion years ago and it's possible that we will have less than 8 planet's before Sun will be ruin.


texts: what is the time mentioned after date (1962-07-03+02:00) the +02:00?

texts:
i was retrieving a some personalities birthday date through computer programming

when i retrieve a person's birthday the result was

1962-07-03+02:00   [this is tom cuise's birthday for information]

In the above result i could understand his birthday is on 1962-07-03 ,

But what is +02:00 , is that the location of birth or GMT ?

for more information the data is retrieved from the URL

http://dbpedia.org/page/Tom_Cruise

sorry to ask this question here , I asked this question because it deals with time and i believe astronomers can help with this


texts: Retrieving input effective radius from Sérsic profile

texts:
I'm playing around with Sérsic profiles, and would like to retrieve the effective (half-light) radius that I put in by integration of the profile. I'm not managing to, so perhaps someone can help me out.

I create the profile using astropy:

from astropy.modeling.functional_models import Sersic1D
from scipy.integrate import simps
Sersic = Sersic1D(amplitude=1,r_eff=1,n=1)

I can then make a light profile:

x = np.arange(1000)
I = Sersic(x)

Then we can perform an integral over this I(r):

integral = [simps(I[0:i],x[0:i]) for i in np.arange(len(I)-1)+1]

From which one would finally find the radius at which the enclosed flux is half that of the total:

from scipy.interpolate import interp1d

f_int = interp1d(integral,np.arange(len(I)-1)+1)
print 'Half radius: ', f_int(integral[-1]/2.)

However, this does not give me the desired result -- the input radius of 1. Why not? Is this a conceptual error or a numerical one - or both?


texts: Is the Milky Way seen as how it looked millions of years ago?

texts: The distant an interstellar object is, the more time is required for light to travel, which is known, measured as light years. So are the farthest stars and gas clusters in the Milky Way seen as how they looked millions of years ago & not how as they look today?


texts: Is it safe to watch solar eclipse's reflection in water?

texts:
There will be a solar eclipse soon at my area... naturally I want to watch this with my son.

Is it safe for us to watch the solar eclipse's reflection in the water? There is a swimming pool near my house and I plan to watch it there.. is it safe to do so?

I remember vividly doing this with my dad when I was little, but some website says we are not supposed to watch it in a bucket of water..

So is it ok to watch it in the water?


texts: Deriving phase of a pulsating star from list of Julian date values of observing brightness

texts:
I want to convert the list of Julian date values of observing brightness of a Cepheid variable into its phase.
The equation I found in the Internet is the following:

$$\mathrm{Phase}= \frac{\mathrm{HJD}-T_0}{P} - \mathrm{INT}\left[\frac{\mathrm{HJD}-T_0}{P}\right]$$
What is the $\mathrm{INT}$ in this equation? I only know that $T_0$ is the one reference point among the list of Julian date values and $P$ is the period.

Also, what kind of criteria exist in determining the reference point?


texts:
The abstract of the new Nature paper Astrophysical detection of the helium hydride ion HeH+ tell us that infrared spectroscopy from SOFIA detected

...rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027.

It also says:

In this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen.

Question: Why was helium hydride (HeH+) the universe's first molecule? Why was it the hydrogen helium cation rather than the dihydrogen cation?


texts:
In the book "Horizons_ exploring the universe-Cengage learning (2018)", p196, it states that

The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium. Because main-sequence stellar cores are
cooler than 100,000,000 K, the helium can’t overcome the Coulomb barrier to fuse in nuclear reactions, so it accumulates at the
star’s center like ashes in a fireplace.

But if the temperature is not high enough to fuse helium, then how does the ending stage of a main sequence star is by burning its helium shell? Is fusing different from burning? If so, why doesn't a helium core star burning its core, even if it cannot reach the temperature about the Coulomb barrier?


texts: Are stars expected to become dimmer before a supernova?

texts: With the recent news about the "fainting" of Betelgeuse and the speculation that this might be a precursor to a supernova, I'm wondering if there is any theoretical/observational basis for this interpretation, or whether this is a case of reading too much into a greater-than-average change in an intrinsically variable star. Why would a star become dimmer in the run-up to a supernova?


texts:
From Wikipedia BLC1:

A paper by other astronomers released 10 days before the news report about BLC1 reports the detection of "a bright, long-duration optical flare, accompanied by a series of intense, coherent radio bursts" from Proxima Centauri also in April and May 2019. Their finding has not been put in direct relation to the BLC1 signal by scientists or media outlets as of January 2021 but implies that planets around Proxima Centauri and other red dwarfs are likely to be rather uninhabitable for humans and other currently known organisms.

BLC1 the candidate SETI radio signal was observed on April 29, 2019, during the period when Proxima Centauri was active violently. These stellar flares and radio bursts are unlikely to generate narrow-band signals themselves. But is it possible that these events trigger some other processes that can create narrow-band signals like BLC1?
Related paper:
A Flare–Type IV Burst Event from Proxima Centauri and Implications for Space Weather


texts: Determine full Moon position into sky from Sun position

texts:
Soon a lunar eclipse will take place soon.

I'm not sure if I will be able to see the moon during the eclipse (from 9:30 PM to 11:00 PM), because, I have some building, mountains... in my field of view.

So, I wonder:

On lunar eclipes, will the Moon position at 9:30 PM be the same than the Sun position at 9:30 AM ?


texts: How much starlight do black holes absorb?

texts: Can it be calculated how much light from other stars a black hole of certain sizes would absorb? And how long will it take for the number of active stars to decrease to the point where Hawking radiation would surpass it?


texts: Can Eight Retrograde (seen from Earth) planets occur?

texts:
First of all, for the purposes of this question Pluto is a planet. So the eight planets being considered are Mercury, Venus, Mars, Jupter, Saturn, Uranus, Neptune and Pluto.

My question is: can all eight of these planets go retrograde† at the same time? I know that seven can do so on rare instances; there were 12 days of that in the 20th century all in the 1940's and the 1980's. But to show how rare that is there was a block of 400 years in the middle ages with no days of seven retrogrades. Has eight ever happened? Or is it impossible?

†Retrograde motion as seen from Earth, not retrograde orbits around the Sun.


texts: How long would a photon take to reach us if it's emitted from a galaxy receeding at c?

texts: A galaxy at the boundary of the Hubble sphere is receeding from us at the speed of light, right? If it emits a photon now, how long will it take to reach us?


texts: Solar Analemma - but with a star instead

texts:
We (homeschool students and I) are trying to find the answer to this question.  Does a star also create Solar Analemma figure 8 when photographed daily for a year?
Has anyone published an experiment where the same star is photographed every night at the same time?
We tried answering this ourselves by googling it, but only come up with websites mentioning the sun.   Is this because it is only possible with the sun?


texts: What's the difference between semi-major axis and apoapsis?

texts:
The semi-major axis is half of the diameter of the major axis of an ellipse by definition. So how does that make it any different from apoapsis, the furthest point an object is from a parent body in its orbit?

Both are radii along the longest axis, surely.


texts: Which telescope's model is being shown to Albert Einstein in this file footage? What event might this be?

texts:
In the Economist video Why does time pass? there is some file footage of Albert Einstein attending some event where a model of a large telescope is on display, and being described to Prof. Einstein.

Which telescopes model is being shown to Prof. Einstein in this file footage? Is it possible to tell what event this might be, or who some of the individuals might be (any famous Astronomers)?


texts: Temperature of Earth if tidal locked with the Sun?

texts:
Assume the Earth somehow stopped spinning once every ~24 hours, and instead started rotating at ~365 days per rotation, so it effectively would end up being tidal locked with the Sun.  In this scenario, the same side of the Earth would always be facing the Sun, and the opposite side of the Earth would always be in darkness.  Disregard all sorts of cataclysmic events that would transpire if something like this happened.

I think it would be safe to assume that life as we know it would quickly cease to exist except on the very edges of the day/night, but what would the approximate temperature stabilize at both on the point on the Earth directly facing the Sun, and a point on the surface of the Earth exactly away from the Sun?


texts:
Any time a spacecraft comes in close proximity to a planet and if the spacecraft has the right angle then it is able to use the planet's velocity to move itself further into space.

According to Newton's 3rd law: every action has an equal reaction.

In this case when the spacecraft uses for example the Earth's gravity to speed up, Earth will move towards the spacecraft. Earth's orbital change will be very small because the spacecraft's mass is small compared to Earth's mass but what if a big asteroid comes in close proximity or what if we use Earth's gravity to catapult our spacecrafts and keep doing it for an extended period of time.

What could happen in this case? Could that have a dramatic impact to Earth's orbit?


texts: Where to get rate of change for calculating ephemeris from JPL Horizons

texts:
I have a working implementation of Schlyter's method for calculating the Sun's position.   I would like to know how Schlyter got the 3.24587E-5_deg in the following calculation for Mercury.

N =  48.3313_deg + 3.24587E-5_deg   * d    (Long of asc. node)

I could not find this number in JPL Horizons.   I'm guessing it's the difference between the N at two epochs divided by number of days between the two epochs, but I'm not sure.


texts: What is the average color of the universe?

texts:
Our Earth, being a beautiful blue orb, has an overall perceived wavelength of somewhere near 470 nm, that being the wavelength of light blue, because the oceans play an enormous role in the weighted average of all of the Earth's colors.

But what about the whole universe? The Earth is nothing in comparison to the host of multicolored stars, ranging from deep red to fiery blue. Taking into consideration the number of stars and each separate color's weighted average,  could we calculate the average color of the universe?


texts: Where can I get a oxygen isotope analysis $^{18}O$ / $^{16}O$ vs $^{17}O$ / $^{16}O$ done commercially ? I witnessed a meteorite fall near me; it looks to be martian andesite. My first number $SiO_2$ 59.32 and my others bring up mars first.


texts: Comprehensible full manual for bolometric light curve reconstruction

texts:
Could anyone point me to some manual or tutorial, possibly step-by-step one, on reconstruction of (quasi-) bolometric light curve using UBVRI-photometry data?

Both links and names are fine for me.


texts: What direction does 0° in the J2000 epoch point towards on January 1, 2000?

texts: Is 0 degrees in the J2000 Epoch 'right ascension' is a line from Earth to the sun on January 1, 2000? Or is 0 degrees the direction towards the ascending node 'equinox'?  Which is NOT towards the Sun on January first...


texts:
For stars, field rotation can be accounted for with an equatorial mount aligned with the Earth's axis and turning at the correct rate.

Since the Moon's orbit is on a different axis then the Earth's rotation it seems like the Moon's own motion would also contribute to field rotation with respect to the observer.

Is this correct? For instance, would pictures taken on a properly aligned equatorial mount still appear rotated to each other over time?

How would we find the field rotation due to this?


texts: What other hints of possible life in Venusian atmosphere have we dismissed?

texts: Now that phosphine has probably been discovered in relatively large quantities in the Venusian atmosphere, I heard that there have been other unexplained phenomena (i.e. dark patches) seen. Does anyone here know of reliable sources with such information?


texts: Finding Observer Latitude/Longitude - In Simple Terms

texts:
I was wondering if someone could explain to me in simple terms how to determine the observer latitude/longitude at a given location. For instance, if an observer is positioned at Cleveland, Ohio, how can I find where the sun and moon is at a certain time of the day?
I am familiar with latitude and longitude but I always retrieve that from Google. Is this the same as those Geographic coordinates or are the observer values different?
Thanks ahead of time.


texts: What are the benefits of a large refracting telescope?

texts:
Currently, refractor telescopes are limited in aperture because gravity will deform the main lens over time. According to Wikipedia the largest practical refracting telescope is 1m.

If we were to overcome this limitation, what would be the theoretical benefit of such a telescope that other types of telescopes currently can't give us?


texts: What constellation does sun occupy by location and date?

texts:
How can i find the location of the sun and which constellation it would occupy based on the location and date of year.

for example: an observer is in Des Moines, Iowa on April 10th, which constellation does the sun appear to occupy?


texts: Would it have been possible to send a radio signal towards ʻOumuamua?

texts:
As we could not get any radio signals from asteroid ʻOumuamua, couldn't we have sent a powerful radio signal to it and then check if we can get any radio signal in response?
This way we can at least check if it is just a rock or any alien spaceship. If it were an alien spacecraft, then they should definitely give a radio signal in response I believe.


texts: Why did the then PhD student Didier Queloz get the Nobel prize?

texts:
When Didier Queloz did the work that earned him a Nobel prize (along with Michel Mayor), he was still a PhD student.

Jocelyn Bell Burnell stated

  I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases

What made this case so exceptional that he got the Nobel in spite of having been a PhD student at the time? Why wasn't the Nobel just awarded to his supervisor, Michel Mayor?


texts: Delta-v from Mercury surface to Venus surface

texts: What it says on the tin: using the most efficient launch opportunity afforded by orbital mechanics, what delta-v would you have to impart to a payload to send it from the surface of Mercury to crash on the surface of Venus?


texts: At what depth below the Sun's surface does the density reach that of water?

texts:
I'd like to write a short supplementary answer to A probe floating in Sun's surface but in order to do so I need to know at roughly what depth relative to the Sun's surface the mass density reaches 1 g/cm^3.

At the bottom of the photosphere the density seems to be either 1E-06 or 1E-09 g/cm^3 as discussed in the question What is the density profile within the Sun's photosphere? but how deep do you have to go below that to reach 1 g/cm^3?


texts: Why are gravitational perturbations stronger at larger semi-major axes?

texts:
Why are mechanisms such as the Kozai-Lidov effect more prominent at at large semi-major axes?

If we had a binary system of a primary and secondary body, with the third perturber as the Sun. Why is it that the secondary feels more perturbations from the gravitational influence of the sun the further away it is from the primary?


texts: What are the most recent minerals modified by water found to date on Mars?

texts:
There likely was quite a bit of liquid water on the surface of Mars less than 4 billion years ago. We know this because we found rocks that old that have been modified by the presence of water. For example :

If the martian blueberries, or hematite spherules found by Opportunity formed by slow evaporation in mineral-rich liquid water, they likely formed 3 billion years ago.
Serpentines have been detected by CRISM.

What is the most recent rock we know of on Mars that was modified by the presence of liquid water on the surface?


texts:
Even though Pluto is not known as a planet anymore, theroetically it has/had a moon, called Charon. I've heard about something that their size are so close to each other that while Charon rotates around Pluto, it also rotates around a central point.

How does it work exactly?


texts: Launch windows for Venus transit; how to convert heliocentric longitudes to date?

texts:
Being a bit familiar with celestial mechanics, I know that the Hohmann transfer orbit is the quickest way to transfer between two circular orbits of different radii around a central body in the same plane. JPL's Let's Go to Mars! Calculating Launch Windows explains the geometry and offers a very intuitive guide how to calculate the travel time to Mars, and this can easily be repeated for Venus.
Just for fun, I would like to mark all launch windows for a travel to Venus on a calendar, similar to the list compiled by Don P. Mitchell
1991, Jun 5     Unused Type I window
1993, Jan 5     Unused Type I window
1994, Aug 9     Unused Type I window
1996, Mar 20    Unused Type I window
1997, Oct 15    Cassini flyby

Question: I have a probably minor (astronomical) issue: I am struggling how to convert heliocentric longitudes into real dates? I guess I am missing the English terms here. I would appreciate a small reminder how to proceed.


texts: Has Hubble ever been used to try to image a near Earth asteroid?

texts:
This answer points out that the near Earth asteroid Apophis will likely be close to 2 arcseconds in diameter as seen from Earth during its close approach in 2029. I speculate that if the Hubble Space Telescope were still operational then, it could potentially image the asteroid in visible light at a few dozen pixels in diameter.

This leads me to wonder if the Hubble has ever been used to image† or at least spatially resolve in some way an asteroid during a close pass to the Earth before.

†Here the verb "image" should be taken to mean the act of producing a resolved image of an object so that different pixels correspond to intensity from different parts of the body being imaged. For the purposes of this question please don't consider telescope images in which an asteroid happens to appear but is too far away to be resolved. Thanks!


texts: How many solar system objects that can really qualify as a KBO are likely to enter Pluto's Hill sphere each day?

texts:
A comment below Which celestial body is able to come closest to Pluto? says that

Technically, many, many small Kuiper belt objects enter Pluto's Hill Sphere (or sphere of influence SOI) every day.

Pluto is small, but it is very far from the Sun, so maybe its Hill Sphere could be surprisingly large, and if we called each particle of dust out there a "small Kuiper belt object" then this can be true.
Question: But realistically, how many solar system objects that can really qualify as a KBO are likely to enter Pluto's Hill sphere each day?
For the purposes of this question let's see if there is some way to ascertain what a KBO might be, and not focus on the helpful comment too much.


texts: Can I have a career in observational astronomy if I study astrophysics or does it have to be particularly astronomy?

texts: So I'm in year 11 and have an aspiration to be an observational astronomer but I'm worried that if I study astrophysics I won't be able to be an astronomer in particular. It probably sounds like a really dumb question. I am open to doing any job in astronomy/astrophysics but I just need to know. Thanks :)


texts: Is the Sun in our solar system moving or stationary?

texts:
When I was small, I read that Sun is fixed at the center of the solar system and that all the other planets rotate around it.
But later I heard that even the Sun is not fixed; it moves. Is this true?
Why had people previously thought that the Sun is fixed?
Is this correct planets in helix path or spring like path?


texts: What can the average Astronomy Enthusiast do to learn more

texts:
Maybe this sounds like a silly question (and hopefully is legitimate), but I will elaborate a little more.

I seem to love astronomy, every single thing about it, I find it brilliant, interesting, mysterious, insanely amazing and mind boggling.

There is only so many documentaries you can watch, read articles on the web etc before you start to see repetition.

I just wondered if there was some advice of what I could do to keep that interesting fire burning...because I absolutely love it!!


texts: How can an amateur astronomer verify the position of near Earth objects?

texts:
Sometimes, hobby-astronomers use rather professional means to observe the big voids of space. Every now and then (think in months, not days) even I can locate an NEO (near Earth object).

Now, I'm assuming governments and institutions worldwide track such NEOs, have the best options to observe them, and most probably accumulated more data than we could wish for.

For hobby-astronomers who spot an NEO, it could be of interest to verify the location of the spotted NEO, and maybe even learn more about a specific NEO from additional data like name, type, path, direction, speed, first observed, etc.

Is there any publicly available, NEO-related database out there? Or is there a specific institution a hobby-astronomer can/should turn to to be able to learn more about individual NEOs?


texts:
I have 2 questions related to spiral galaxies.

Firstly, how did the arms form? Why would the stars accumulate into those specific areas? And secondly, why are they still intact?

The inner parts of the arms must be revolving more rapidly than the outer parts, so surely the would become stretched and then no longer resemble arms, but simply a disk around the galactic center.


texts:
I am trying to determine the lunar surface near or far side daytime illumination levels in Lux or foot-candles but there seems to be no directly measured values available. I did find this information on the levels as measured from earth, so it should be possible to work backwards to arrive at those values I presume. My math is very rusty indeed though, so thought I'd put the question to some experts.
How bright is moonlight?
https://academic.oup.com/astrogeo/article/58/1/1.31/2938119


texts: What is the longest observable wavelength of light using an optical telescope?

texts: How far outside the visible spectrum could an optical telescope feasibly operate?


texts:
I am trying to generate skymaps for a set of Fermi GBM triggers, containing the probability that a trigger originated in a given part of the sky.  For each trigger, I have access to the RA, dec, error radius, and reliability values.

This website is a perfect example of what I am trying to do.  There is a table with with RA, Dec, error, and reliability values, which were used to generate maps (sample).  My question is, how was this done?

My understanding of the problem is that I must first generate fits files based on the data, and then use either a python library or external program to render the skymap from the file.

I have already looked at both healpy and astropy, which provide a means to create and manipulate fits files.  Creating a blank fits file is easy, but I am at a loss as to how to populate the files I generate with relevant probabilities based on the data I have access to. Any help, even just a pointer in the right direction, would be greatly appreciated. Thank you for your time.

edit- description of the error radius parameter:

  This parameter is the uncertainty in the position, in degrees. The GBM errors are not symmetric, and the given value is the average of the error ellipse.

This is what results in the asymmetrical blob shapes.


texts: Could someone explain RA/dec in simple terms?

texts:
The equatorial coordinate system isn't too complicated. However, for people new to this, it seems rather intimidating at first.

Is there an easy way to explain it?


texts: Planned telescope to detect alien waste-heat

texts:
I remember watching a talk by someone who designed a telescope to view a single star at a time.

The purpose of the telescope was to detect waste-heat which would presumably be emitted by any civilization utilizing a large percentage of their planet's solar energy, due to the second law of thermodynamics. By focusing on a single star, it would be able to detect when the planet crossed the path of star-light, and the additional energy emitted by the planet - or something like this.

Anyone know who this might be?


texts: Is it possible to build a ground-based telescope that can directly image Earth-like planets orbiting in the habitable zone of sunlike stars?

texts:
In the following discussion I will refer to Earth-like exoplanets planets orbiting in the habitable zone of sunlike stars simply as “Earth-like exoplanets."

NASA is currently developing or considering three space telescopes that would be able to directly image Earth-like exoplanets:

The WFIRST space telescope (if it’s equipped with a starshade).
The HabEx space telescope, which would be equipped with both a starshade and
a coronagraph.
The LUVOIR space telescope, which would use a coronagraph to block the light
of an exoplanet’s star.

I can think of three possible reasons why it might not be feasible to build a ground-based telescope that can (like the three space-based telescopes) directly image Earth-like exoplanets:

Reason 1: A starshade is one of the best technologies for blocking the light
         of an exoplanet’s star, and it seems very difficult to use a
         starshade with a ground-based telescope.

Reason 1 seems valid, but it doesn’t seem that it can be the whole story
because (as discussed above) the LUVOIR telescope will only use a coronagraph
to block the light of an exoplanet’s star, and the HabEx telescope will use a
starshade for some observations and a coronagraph for other observations.
Reason 2: Maybe the type of coronagraphs planned for the LUVOIR and HabEx
         telescopes wouldn’t work for a ground-based telescope.
Reason 3: One of the main reasons why scientists want to directly image
         Earth-like exoplanets is to spectroscopically examine their
         atmospheres to look for possible signs of life such as oxygen and
         methane. Maybe spectroscopically examining the atmospheres of
         Earth-like exoplanets requires imaging in the infrared spectrum,
         which isn’t possible for ground-based telescopes.

Are these three reasons valid, and do they imply that it isn’t feasible to build a ground-based telescope that can directly image Earth-like exoplanets? Are there other factors that need to be considered? Or is it actually possible to build a ground-based telescope that can directly image Earth-like exoplanets?


texts: I'm making a simplified animation for college project on how a star reacts with a black hole. I had known that a black hole sucks in everything and thus should be doing the same for stars. But upon my research I found out that recently a discovery was made that a super-massive black hole ripped apart a star. Now I've always been curious about cosmology and this took my attention and also worried me for my assignment. There were two conditions with the above mentioned calamity - one, the black was a super-massive one and second, the victim star was possibly already stripped off it's outer gaseous by the same black hole. My question is that the ripping apart happens in all conditions and the sucking in thing was wrong or it happens only in mentioned circumstances?


texts: What if a Black Hole passed through an Irregular type galaxy?

texts:
So, most of you all are probably (well, why wouldn't you be) familiar with The Magellanic Clouds, and hour they are Irregular type galaxies. You should also be aware that they both orbit the Milky Way, so they act sort of like moons to a planet. Since they (most likely) do not have a central Supermassive Black Hole, I wonder, out of all ridiculousness, what are the answer to these two questions?:

1) What would happen to the Small or Large Magellanic Clouds if a Black Hole (maybe 100 time bigger than the sun) passed though them?

2) What would happen if Sagittarius A* passed through either the Small or Large Magellanic clouds. If you are not aware of Sagittarius A*, it is the Supermassive Black Hole that lies at the center of the Milky Way.


texts: Just how "locked" are resonant-chains of exoplanets thought to be? (e.g. K2-138 and TOI-178)

texts:
update 2: A&A: Six transiting planets and a chain of Laplace resonances in TOI-178 (and arXiv)

Gizmodo: Enigmatic Star System Has 5 Planets Locked in Perfect Harmony
Bad Astronomy A Six-Planet System Dances in Time to the Tune of Gravity

update 1: The BBC News article Citizen science bags five-planet haul announces the system K2-138, also described as a "resonant chain" of planets. However, see the actual paper in ArXiv and Ast. J. where the title calls it only a "near-resonant chain" instead; The K2-138 System: A Near-Resonant Chain of Five Sub-Neptune Planets Discovered by Citizen Scientists
The Phys.org news item Discovery of new planet reveals distant solar system to rival our own outlines the recent announcement of results using AI to help search Kepler photometric (transit-method) data for exoplanets. Near the end is the paragraph:

Kepler-90i wasn't the only jewel this neural network sifted out. In the Kepler-80 system, they found a sixth planet. This one, the Earth-size Kepler-80g, and four of its neighboring planets form what is called a "resonant chain," where the planets are locked by their mutual gravity in a rhythmic orbital dance. The result is an extremely stable system, similar to the seven planets in the TRAPPIST-1 system, so precisely balanced that the length of Kepler-80g's year could be predicted with mathematics.

My question is about these "resonant-chains" of planets. If I understand correctly this would be a group of planets where due to mutual perturbative effects they are in orbits who's periods are in mutual rational number ratios, e.g. 3:2 or 7:9.
Is it known yet how long-lived (time domain) or narrow (frequency) domain these resonances are, or are likely to be? In other words, are they thought to be really "locked" into these fixed ratios for say tens or even hundreds of millions of orbits, or are the orbits just really close, but with occasional, irregular "slippage" events?
Since the span of data and its precision is limited, I would expect that one can not conclude from the data alone, and it's likely some modeling has been done on resonant-chains of planets to improve the understanding of the phenomenon, so I'm asking to understand what's known and believed, rather than a precise answer about a given system. Since the problem is mostly orbital mechanics it lends itself more easily to simulation than most problems.
As background, Wikipedia addresses "locked" orbital resonances:

Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. (emphasis added)


texts: Period of pulsation of a star and its relation to free fall and acoustic waves

texts:
In this paper, the period $\Pi$ of pulsation for a star can be obtained by means of the virial theorem:

$$\Pi = 2\pi\sqrt{\frac{sR^3}{(3\gamma - 4)qGM}}$$

where $s$ and $q$ are constants relating to the star and $\gamma = c_P/c_V$ is the ratio of specific heat capacities at constant pressure and volume. I would like to know how one can relate $\Pi$ to:

(a) the time of free fall

and

(b) the acoustic travel time

from the surface of the star to its center.


texts: What percent of planets are in the position that they could be viewed edge-on from Earth? (and thus able to undergo transits)

texts:
Star number 12644769 from the Kepler Input Catalog was identified as
  an eclipsing binary with a 41-day period, from the detection of its
  mutual eclipses (9). Eclipses occur because the orbital plane of the
  stars is oriented nearly edge- on as viewed from Earth. During primary
  eclipses, the larger star, denoted “A,” is partially eclipsed by the
  smaller star “B,” and the system flux de- clines by about 13%

From http://www.sciencemag.org/content/333/6049/1602

Here's the thing though: out of all possible edge-on configurations, there are far more configurations where a planet can never be in the position to be edge-on, rather than configurations where a planet could potentially be in the position to be edge on. (I'd guess that it happens in fewer than one in several hundred cases)

So why are we able to observe so many transits?


texts: How are the compositional components of exoplanet atmospheres differentiated?

texts:
How are exoplanetary atmosphere compositional spectra distinguished from those of the parent star(s), from the composition of the planetary surface or any other factor?  Is it actually possible to determine the atmospheric composition precisely using this method?

Are there any specific examples of such an analysis having been performed on an exoplanet?


texts:
In a matter of weeks, I will be performing my first observation using a 25-cm Newton telescope (+CCD camera) and would be grateful for advice regarding an otherwise basic research that I have decided to try out due to this occasion.

I will be observing eclipsing binary stars (type Algol), yet have never used any program or site. Just to determine which binaries will be visible at my location on a specific date at a specific time - which sites do you use for this?

Also, what should be my main goals when taking photos in terms of timing and exposition?

Later on, I will use APT for photometry and estimate the time length of the eclipse, which will be my objective.

Thank you in advance.


texts: Cases of gravitational lensing resulting in a recognizable image of an extended object?

texts:
There are several different classifications of gravitational lensing phenomenon.

Here I am asking for any examples of strong lensing where the lensed image of an extended object is magnified and recognizable as an extended object. Of course it may be distorted somewhat, but if the lensed object is a galaxy, then the lensed image should appear larger and "galaxy-shaped" even though distorted. If it's a tight pattern of stars or other objects, then the pattern should be larger.

I have a strong memory of reading about such a case in Nature or Science within the last few years, but I can not find it. I believe that the nearer, lensing object was a galaxy.


texts: Is it possible that the Sun has a binary partner (the Nemesis Theory) that has eluded detection?

texts:
I just recently learned about the theory that our Sun has a small companion star with a 26 million year orbit. This theory came about when it was realized that mass extinction events happened approximately every 26 million years. My question is not whether or not this is possible as that has been asked before; my question is how could we have missed it?

When we look for exoplanets in the Universe, we are sometimes able to detect a slight wobble in the parent stars. Along those lines, if the Sun had a companion star, how would we not notice the effects on its rotation/orbit? If the sun and this companion star were locked in a 26 million year orbit, would we be able to notice the orbit?

Finally, when you have a star system like ours, with so many planets, is it possible to tell which planet is causing which specific motion in the Sun? For example, let's say we traveled to a different star system a few hundred light years away, and we were studying our solar system. Would we be able to look at the way the Sun wobbles and determine how many planets were in orbit around our Sun, or would we only be able to say that there was one, or simply that there may be several? I guess, the root question is how accurately can we judge the contents of a star system based on the perturbations in the star's orbit? Thank you kindly for your time.


texts: Why don't stars move in the night-sky as the moon does?

texts:
I want to know why don't we see a change in the position of stars during the night while we do see a change in the position of the moon. I've checked other online sources and some answers say that it is because they are too far away, is this true?

I mean for example if i'm laying down on the earth since sunset looking at the stars, and let's say I'm close to the equator, no matter the distance, I'll be looking at the stars and earth will turn about 180° until sunrise, so my angle of view will be about 180°.


texts: How many space telescopes are currently active?

texts:
Hubble, Chandra, XMM-Newton, Kepler,... I can name a few off of the top of my head, but how many space telescopes are there in all?
Related

What are the next planned space telescopes?


texts: In the closest stars, can you see the stellar corona?

texts:
Stars appear larger than they are, could that be the stellar corona here around Sirius, making it appear n-times larger than the stellar body? NASA recently mapped the solar corona to extending 12 solar radii above the surface 1, the corona as a whole occupying 13x the suns diameter if viewed from a distance.

2

Refences:

1) NASA's STEREO Maps Much Larger Solar Atmosphere Than Previously Observed - nasa.gov


texts: How/where to check where the Sun is (constellation)?

texts: I would like to check in which constellation the Sun was in at some time in the past, any recommendations how to do that?


texts: Does natural satellite(s) of a planet affects its orbital velocity around a star?

texts:
Please excuse me for a lay question.

As we know, to calculate the orbital velocity, we take the mass of the orbiting body, the mass of the body being orbited and the distance between the two bodies into account.

However, we don't really care about the natural satellite(s) (of a planet) in the calculations. Why are they insignificant? Why isn't the whole system involved in the calculations?


texts: Is the discovery of a "mass gap" black hole by this tiny telescope just plain luck/serendipity or could it be the first of many similar discoveries?

texts:
The Kilodegree Extremely Little Telescope is a pair of modest lenses with high quality medium format CCDs attached sitting on oversized mounts in Arizona and Sutherland.
These very modest instruments have discovered several exoplanets (by choosing a niche magnitude range) and have now seemed to have discovered a particularly interesting 3 $M_{\odot}$ black hole + red giant binary.
Jayasinghe et al. (2021) A Unicorn in Monoceros: the 3M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate
Question: Is the discovery of this "mass gap" black hole by this tiny telescope just plain luck or serendipity, or could this be the first of many similar discoveries? How likely is it that there are more nearby "mass gap black holes" that just got missed due to the parameter space sampled by time domain based surveys?

From keltsurvey.org/telescopes

This picture (KN) shows the KELT-North telescope at Winer.

This picture shows the KELT-South telescope at Sutherland.

<img src="https://i.stack.imgur.com/5EEgQ.png" alt="Figure 13. The right panels show the template subtracted H (red dot-dashed), H (blue dashed) and Ca i 6439 (black) line profiles at various orbital
configurations illustrated in the left panels. The size of the compact object is simply chosen to make it easily visible." />
click for larger

Figure 13. The right panels show the template subtracted H (red dot-dashed), H (blue dashed) and Ca i 6439 (black) line profiles at various orbital
configurations illustrated in the left panels. The size of the compact object is simply chosen to make it easily visible.


texts: Photometric redshift - empirical methods (machine learning)

texts:
I am currently learning about estimation of photometric redshifts with machine learning methods (or empirical methods in general).
These methods use the knowledge about the photometry and the spectroscopic redshift of many galaxies in order to infer a mapping between the photometry and the redshift. Then, based on this mapping, redshifts can be estimated for the photometry of other galaxies.

I've read that for empirical methods it is crucial that the training data (i.e. the data from which the mapping of the photometry to the redshift is inferred) represents the galaxies for which estimated redshifts are desired in the future. I understand that this is crucial, but in what sense does the training data represent a certain distribution and in what sense are other galaxies represented or not represented by this training data? How do I know if a galaxy is well represented by the training data so that I can estimate a redshift for the galaxy?

Would the galaxy have to be from the same region? Does it have to have the same mass? What are the factors to look at, if I want to know whether a galaxy is from the same distribution/is well represented by the training data?


texts: During an eclipse, does the size of the moon and sun match perfectly?

texts:
During a lunar or solar eclipse, from our perspective when we look towards the moon and the sun is behind the moon, does it fit exactly?

Has anyone calculated the sizes of each in a relative context?  Obviously the sun is thousands of times larger, but I am talking about the size of the sun/moon when looking from our perspective.

I hope my question makes sense.


texts: Need help understanding the recent observation of extra-galactic planets!

texts:
The new paper Probing Planets in Extragalactic Galaxies Using Quasar Microlensing (also in ArXiv) is too mind-boggling for me to understand exactly what is happening.

I have some very basic understanding of gravitational microlensing from other Q&A here (and Wikipedia) but I can't figure out what is happening in this paper.

There are thousands of planets seen in another galaxy, which do not orbit stars? There is discussion of "emission close to the event horizon of the supermassive black hole of the background quasar"... there are too many unusual things going on at once for me to fathom.

Is it possible for someone to sort out the basics of what's happening? The Quasar is in front, and the galaxy containing these planets is behind? What emission?


texts: Does the earth spiral around the sun's movement/motion path?

texts:
I have watched the following video (How Earth Moves by Vsauce) regarding how earth moves:

Here are some screenshots:

I have some questions:

Does the earth spiral around the sun's path as it is shown in the video (exact time is 19:49) and screenshots? Is the spiraling movement caused by sun movement (on its path shown above) where the sun is dragging the earth (and other planets).
Does the earth go in front of the sun then back, then fourth and back like it is shown on the video? I see both the sun and earth racing with each overtaking the other over and over. Is this assumption true? In this manner, would earth, at one point in time, be closer to the destination where the solar system is heading (I think they call the destination Vega) than the sun?
I am working on a poetry piece. If I mention that earth and sun are locked in a never ending race with each taking the lead (earth moving in front of the sun's path / winning) and earth actually dropping out of the race (spiraling/moving out the race and falling behind), that they are destined to never meet (collide), would I be correct?

P.S. My knowledge of astronomy/physics is very limited. I have tried my best to be as clear as possible. Sorry for any confusion. Thanks.


texts: What does the word "shock cooling" mean in supernova physics?

texts:
I cannot understand the meaning of "shock cooling" which occurred in supernova.
And I saw the sentence like "Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope". Could you explain the mechanism of shock cooling in supernova?


texts: Why is the moon more visible during the winter?

texts: I was curious to find out that as to why, the moon looks more sharper during the day in winter, whereas it's almost not visible during summer.


texts: If the whole observable Universe is as big as planet Earth, how big would Earth be?


texts: Do secondary stars in binary systems affect how the habitable zone is calculated or they can be calculated separately?

texts: I was taking a look at this tool to calculate habitable zones: Calculation of habitable zones which is pretty simple: you need to input Teff in Kelvins and Stellar Luminosity in solar units. Then I was wondering if this calculation is affected when you have binary systems and the tool just doesn't work anymore, or each habitable zone can be calculated separately. Do the effects of both stars add up and alter the calculation?


texts: LIGO: How can laser interferometry (wavelength >$10^{-7}$m) detect length changes of arms <$10^{-18}$ m?

texts:
I'm trying to understand the sensitivity of the LIGO interferometer. I've been reading around lots of discussion of how they manage noise cancellation between the two detectors, achieving a very pure laser signal, the many reflections to change the effective length of the interferometer arms to over 1000km, and other impressive tricks of engineering to achieve this remarkable feat of measurement. What I still can't get my head around is how, even in a perfect world with no noise at all, a phase shift of I guess $10^{-7}$ - $10^{-11}$ cycles or so (depending on the sensitivity claim you are taking) can show up as a signal.
From my fairly basic understanding of how an interferometer works. I am imagining that if both beams are initially in phase then a $10^{-7}$ phase shift would change combined amplitude by much less than this (1$0^{-14}$?) If they were at $\pi / 2$ then I guess the amplitude change would be approximately equivalent to the phase shift?
I appreciate that the computational details of how all this works are going to be a whole lot more complicated, but I would really appreciate some pointers on how best to understand the sensitivity challenge here:

Is it a case of measuring tiny amplitude variations in the combined laser signal?
Is the size of these amplitude variations (as a proportion of amplitude) equivalent to the size of the length variations (x number of reflections and as a proportion of laser wavelength)?
If yes to each of the above, am I right in thinking they are detecting laser amplitude oscillations down to $10^{-10}$ ish times typical amplitude?
If no to the first two points, please set me straight in any way you can!
Many thanks in advance.


texts: Calculating object velocity at perihelion

texts:
I would like to calculate the velocity of an asteroid orbiting around a star (Sun) at the perihelion of its orbit. I know the excentricity of the ellipse and its semimajor axis.

I have found that the vis-viva equation is used to calculate the velocity of an object on an elliptical orbit and that the perihelion is at distance r = a(1-e). However I (simply enough) cannot see how to mathematically combine these two pieces of information in order to get the velocity at the perihelion.

(I am not so much looking for just a formula but rather a proof/intuition regarding how to get from the vis-visa equation for velocity to a perihelion velocity equation)


texts: How does a Solar Transit give more accurate determination of Earth-to-Sun distance?

texts:
I understand how Solar Parallax calculation determines the Earth-Sun distance using measurements of (a) the baseline distance between two widely-separated terrestrial telescopes and the (b) two angles - one at each telescope - between the baseline and the line from telescope to the centre of the Sun's visible disc.

How is it that observations of the transit of a planet across the face of the Sun give better results than simply pointing (simultaneously) each telescope at the centre of the Sun's visible disc?

Is the Solar Transit method also superior (in terms of accuracy) to the method of determining Earth-Planet distances by measuring parallax angles relative to 'fixed' background stars?  If so, why?


texts: Without using absolute magnitudes or isochrones, how might we tell a star's age and evolutionary status?

texts:
Usual methods of estimating stellar ages involve isochrone approximations. It can also help to estimate a star's radius by correlating its absolute magnitude with effective temperature and apparent magnitude. In the absence of these measurements or observations of tell-tale variability, how might you guess a star's age and evolutionary status?

Given a single, high-resolution $(R\gtrsim 50000)$ spectrum as your only data point, how easy is it to accurately infer the age and evolutionary status of a star? For example, how would the spectrum differ between a red dwarf and red giant, both with of $T_{\mathrm{eff}} = 4000~\mathrm{K}$? Or between two red dwarfs of ages $2~\mathrm{Gyr}$ and $8~\mathrm{Gyr}$?

A good answer could describe how surface gravity $(\log g)$ affects spectral lines (and how this relates to stellar mass and radius), what elements we might observe more strongly at different stages of evolution, and some observational results of gyrochronology.


texts: List of Moving stars on the northern hemisphere? [Stellar kinematics]

texts:
Anyone know if there is a List of Moving stars on the northern Europe hemisphere I can search for? Like Barnard's

I wish to image them [when possible] and compare their yearly movement.

Thank you


texts: Relationship between Dark Energy and Dark Matter

texts:
I have encountered two seemingly contradictory theories on this(I don't know which one of them is correct, or where I am wrong):-

From Wikipedia:

  Measurements of cosmic microwave background (CMB) anisotropies indicate that the universe is close to flat. For the shape of the universe to be flat, the mass/energy density of the universe must be equal to the critical density. The total amount of matter in the universe (including baryons and dark matter), as measured from the CMB spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%."

The above statement, if correct, I suppose indicates that there is a critical density for which mass/energy has to account for. This means that if suppose there was no Dark Matter, our calculations/assumptions regarding the amount of Dark Energy would have been much higher(because then Dark Energy would have to account for a greater share of critical density).
The universe is accelerating in expanding. Dark Energy is supposed to cause this acceleration in expansion. While ordinary matter plus Dark Matter, is supposed to reduce this acceleration/expansion through gravity. So, according to this theory, if there would have been no Dark Matter, then there would be less Dark Energy required to account for the observed acceleration in expansion, so our calculations/assumptions regarding the amount of Dark Energy would have been lower.


texts: Any known Triple Star Systems with this hierarchy?

texts:
I'm curious, are there any known STABLE triple systems in which a close inner binary pair is orbited by a more massive star, yet the total mass of the binary pair is greater than that single orbiting large star?
Something relatively compact, like this:

Close Inner Binary: Star A with mass of 1, Star B with mass of 0.8. Combined Mass: 1.8
Outer Star Orbiting Binary: Star C with mass of 1.2.
Inner Binary Orbital Period: months or a couple years.
Outer Star Orbital Period (orbiting binary): Decades to maybe a century or two.

Star A and B are both smaller than Star C, with the combined mass of A and B exceeding Star C. Star C orbits the barycenter of the close binary A+B.
I'm only aware of triple star systems in which the largest star is closest to the combined barycenter of the system. I don't know of any examples in which the largest star is also the star furthest from the system barycenter. Does this arrangement exist?
Thanks!


texts: Would gravitational waves be subject to external gravitational perturbations?

texts:
Given that the gravitational wave detected by LIGO was a very weak echo of a very distant event, could it have been "deviated" and distorted on its way here by a sufficiently massive black hole in between?

Would a black hole, since it traps even light, and since gravitational waves travel at the speed of light, absorb some portion of the gravitational wave and leave a "shadow" in its propagation such that if a black hole was between your instrument and an incoming gravitational wave you would have no way to detect it?

If so wouldn't the mass distribution in the Universe look less homogeneous at larger scales (as observed), since gravitational waves would not deform space uniformly and would instead need to "equalize" by forming voids like the Boötes void, and filament structures like the Pisces–Cetus Supercluster Complex or Sloan Great Wall?

If not, how could a gravitational wave from a distant event possibly affect the gravity well of a black hole closer to the observer? It doesn't seem possible to me that the attenuated ripple in space time from an event so far away could pass right through the gravity well of a closer local black hole like a galactic supermassive black hole.


texts: Is it realistic to build a telescope powerful enough to take a photo of TRAPPIST-1's planets?

texts: Assuming modern and near future technology, could we build a telescope powerful enough to actually see TRAPPIST-1's planets?


texts: Why do we continue to find a discrepancy in the Hubble Constant?

texts:
After taking a look at some recent papers (examples below) that attempt to derive the value of the Hubble Constant, they all conclude with dramatically different values (speaking in context). Why is this? Each of the techniques seems sufficiently rigorous and equally well rooted in understood physics. Is this simply some form of error or is it due to some yet-to-be-discovered physics?

Papers:

A gravitational-wave standard siren measurement of the Hubble constant, H0 = 70
Planck 2015 Results, H0 = 67.8
New Parallaxes of Galactic Cepheids from Spatially Scanning theHubbleSpaceTelescope: Implications for the Hubble Constant, H0 = 73.48

I do understand that this is a hot topic right now in cosmology, however I just don't yet understand why we're having trouble with getting our measured values of the Hubble Constant to converge.


texts: I am using a convex lens with $f=5$ cm and $5$ cm aperture, a concavo-convex lens with $f=100$ cm and $5.5$ cm aperture. I am trying to make telescope and it should have magnification of 20 x. But the image coming of eyepiece is very small and cant be seen clearly. Please help with the problem?


texts: Why does the closest approach of star S2 to Sgr A* not appear to be near the focus of its elliptical orbit?

texts:
Phys.org's Very Large Telescope sees star dance around supermassive black hole, proves Einstein right links to several ESO videos, including The star S2 makes a close approach to the black hole at the centre of the Milky Way.

In the screen shot below, the location of Sgr A* is nowhere near a focus of the elliptical orbit.

In addition to the excellent accepted answer I have written this answer to Why does Earth not appear to be at the focus of TESS' elliptical orbit in this video? and so I know that looks can be deceiving depending on one's perspective.

But I still don't really understand why the offset can look so bad here. For the video did they put the observer (the "camera") at a fairly close distance and thereby distort the perspective, or even if you choose the distance to be infinite can elliptical orbits appear to not have their central body at their focus, or at least along their axis?

Is it possible to state the orbital elements of S2 in such a way and in some coordinate system so that I can plot them in 3D and then try to reproduce this effect?


texts: Angular distance/apparent separation between AE Aurigae and Orion's belt

texts:
The title perhaps gives the question away, but would anyone know/be able to roughly approximate the angular distance/apparent separation between AE Aurigae and Orion's belt.

I have the value of $42^\circ$, from visual estimation but would like a way to calculate this.


texts: How did Fermi manage to see neutron star merger?

texts: When the GW170817 neutron stars merger was observed by LIGO/Virgo, the Fermi gamma-ray telescope observed the event 2s after the merge. How did it know where and when to look? It must take some time to rotate the satellite and I guess that evaluation of data from LIGO/Virgo also isn't instantaneous. Or is it? And does it automatically send suspicious signals to the Fermi operation centre to point it to the right location?


texts: Why would the Chang'e-4 lander find lunar far side temp. "colder than scientists expected", when the LRO has already been taking thermal readings?

texts:
In January, the Chinese probe lander Chang'e-4 was was announced to have found temperatures dipping lower on the far side than expected ("Chinese rover finds lunar nights 'colder than expected'" by R. Jackson, Jan. 31, 2019. AFP via Phys.org):

  Temperatures on the moon's surface plummeted to minus 190 degrees celsius (-310 degrees Fahrenheit) during the probe's first lunar night, which "was colder than scientists expected," the China National Space Administration (CNSA) said.

This was said to be surprising, because near-side temperature measurements by U.S. missions showed less substantial dips. It was suggested that this was probably due to a characteristic difference between regolith on the two sides:

  They were lower than those recorded by previous US missions to the near side of the moon, Zhang He, executive director of the Chang'e-4 mission, told Xinhua news agency.

  "That's probably due to the difference in lunar soil composition between the two sides of the moon," he said.

But the U.S. surface missions were not the last thermal readings taken of the Moon, and the Chinese mission isn't the first mission to take thermal readings of the far side. Unless the thermal variability measured by Chang'e-4 is particular to a very localized area of regolith on which it took those lunar dawn readings (in which case it can't be said to be representative of a generalizable difference from near-side regolith), how would that not be known already, from the Infrared measurements that have been taken by the Lunar Reconnaissance Orbiter for the past decade,
 with enough resolution and sensitivity to distinguish the thermal variation of craters from surrounding regolith?


texts: How to convert the observed magnitude of a Quasar to it's bolometric luminosity?

texts: How can I convert the observed magnitude to Bolometric Luminosity? I know the relation for the star but should it be same for a Quasar? Basically I want to convert the X-axis of this plot to luminosity (or mass).


texts: Could the Earth survive a red giant Sun?

texts: So my question is that, is not a lie that we are flying away from the sun slowly, that is increasing by time because the sun loss mass, so when the scientist usually talk that we will get swallow by Sun when it get red giant, do they count on this? I mean, we can get far enough to survive or they just calculate with the actual orbit?


texts: Between Mercury and Venus, which planet produces a longer transit? Assuming both planets describe same path on the solar disk

texts:
Is it even possible to give a definite answer to this question? There are a lot of factors involved in this like Earth's position and motion relative to the planets and of course their own motion and apparent sizes in earth's sky.

Or is it that obvious that Venus transit will be longer because its orbital speed is less than Mercury?


texts: Where is the habitable zone of a Hypergiant?

texts:
Where would an Earthlike planet need to be so it's still in the 'Habitable Zone' of a hypergiant Star, compared to the distance of the Earth from the sun?
Could complex life develop or survive on such a planet?

It's not necessary to answer the second question, as it is not really related to Astronomy much. But if someone could really answer that, I would be grateful.
Some background
I was watching this video of Kurzgesagt ( that's a YouTube channel.) which was released only some days ago, and the video spoke about the comparison of the Sun to some of the largest types of stars out there. The video also mentions Giants and of course, Hypergiants.
This made me think about what would happen if our very own Sun, turned into a Hypergiant.


texts: Calculate eccentricity from apside altitudes?

texts:
I need to calculate the eccentricity of a satellite orbit with only the apside altitudes (height above the surface of the Earth), 400 and 1500 km. However, I don't know the semi-major axis ($a$) or semi-minor axis ($b$).

$$\text{eccentricity} = (1-(b/a)^2)^{1/2}$$


texts:
I am making a small orbit simulator. I figured out Kepler's laws and know how to work with them to "update" an orbit based on time. So now I want to know how I can describe orbits with newton laws. Given a position vector and a velocity vector of a satellite, What I currently do is this:

Calculate gravitational acceleration: (g*mass)/distance^2
Add this number to the velocity vector.
Multiply the new velocity vector with the DeltaTime
Add it to the position vector
Go back to 1.

However when converting this to Keplerian Orbit Elements doesn't really make them constant.(Calculations from here: Cartesian To Kepler)

Also the orbit isn't really stable, and also wobbles around. So can anyone help me fix this? Or am I doing something wrong?


texts: What can the total stellar mass of galaxies tell us?

texts:
The total stellar mass of a galaxy cannot be measured directly. Astronomers can infer it from the galaxy spectrum by stellar population synthesis. However, since high-mass stars dominate the spectrum but contribute little to the total stellar mass, the inferred total stellar mass is sensitive to initial mass function (IMF) which is usually assumed a priori. One can also measure the total stellar mass by kinematics, but it is the total mass of stars, dark matter and gas enclosed that is measured in this way, and we still need some assumptions (say, on the dark matter profile) to obtain the total STELLAR mass. In a word, it's very difficult to measure the total stellar mass of a galaxy precisely.

So I wonder why people are still interested in this parameter. What can it tell us about the formation and evolution of galaxies?

It seems that what can be measured relatively more accurately is the properties of high-mass stars which are also the main sources of stellar feedback and metal enrichment. Why not just focus on these stars? What makes low-mass stars (or to be more concrete, their mass) important to the galaxy as a whole?


texts: Do all objects get larger in an "expanding universe"?

texts:
I'm envisioning an analogy of the universe as a plane with marbles in that plane to represent different planets.

Now, if this plane expands how does this work? I assume that it is not like 'nothing' becomes 'something' then but rather expanding/stretching the universe; but doesn't this imply that everything around us is actually stretching/becoming bigger (like every atom/quarks) becomes a little bit bigger? Or is it just the space between those subatomic particles and combinations of these become bigger?

An example could be the structure of carbon dioxide becoming larger, but only because there is more space between the carbon and the oxygen atoms.


texts: Definition of the light that is projected by a celestial body/luminary

texts:
I'm creating an application which will simulate the light created by sun or moon on a 3d topographic map of Earth. Something like this:

Is there a word or definition for this light/light projection?


texts: What's the difference between optical telescopes and solar telescopes?

texts:
Solar radiation is strong. Following problems for solar telescopes may be telescope site selection and temperature control.

I wonder the telescopes themselves and their instruments are different?